Solar cells are devices that convert the energy of light into electricity using the photovoltaic effect. Solar power is an attractive green energy source because it is sustainable and produces only non-polluting by-products. In operation, when light hits a solar cell, a fraction of the incident light is reflected by the surface and the remainder is transmitted into the solar cell. The photons of the transmitted light are absorbed by the solar cell, which is usually made of a semiconducting material such as silicon. The energy from the absorbed photons excites electrons of the semiconducting material from their atoms, generating electron-hole pairs. These electron-hole pairs are then separated by p-n junctions and collected by conductive electrodes which are applied on the solar cell surface. In this way, electricity may be conducted between interconnected solar cells.
Solar cells may have a variety of different structures. Conventional solar cells have an antireflection coating (ARC) applied to the front side of a semiconductor substrate, so as to reduce reflection of incident light. Front side electrodes, typically formed of electroconductive pastes, are applied on top of the antireflection coating in a grid-like pattern. Another electroconductive paste (which may be the same or different from the front side electroconductive paste) is applied to the backside of the solar cell to form backside electrodes/soldering pads. An aluminum paste is also applied to the backside of the substrate, overlapping the edges of the soldering pads, to form a back surface field which improves solar cell performance.
One specific type of solar cell is a passivated emitter rear cell (“PERC”). In conventional solar cells, the aluminum paste is applied directly to the back surface of the solar cell substrate to form the back surface field. In a PERC solar cell, a rear passivation layer, typically formed of a dielectric material such as alumina, is first applied to the back surface of the substrate. Additional rear surface layers, such as a silicon nitride capping layer, may be applied on top of the rear passivation layer. Next, portions of the rear passivation layer and additional rear layers are removed to expose areas of the underlying substrate. This step may be achieved by, for example, acid etching or laser drilling. The aluminum electroconductive paste is then applied on top of the rear layers, thus filling in the “holes” created during the removal process. In these areas, a local back surface field is formed when the aluminum paste is fired. The backside soldering tabs may be applied to the back surface either before or after the application of the aluminum paste. A standard solar cell and an exemplary PERC solar cell are illustrated in FIGS. 1 and 2. PERC solar cells have been shown to have increased efficiency compared to solar cells that do not have a rear passivation layer.
The electroconductive paste compositions used to form the front side and backside electrodes are specifically formulated for their particular application. A typical electroconductive composition contains metallic particles, an inorganic component, and an organic vehicle. With respect to the pastes used to form the backside soldering tabs on a PERC solar cell, they are typically formed using silver as the metallic particles, glass frit as the inorganic component, and an organic vehicle. These PERC backside pastes are designed to have limited or no chemical reactivity with the underlying substrate layers, such as the rear passivation layer and capping layer, so as to avoid damage to those layers which results in decreased electrical performance of the solar cell.
Accordingly, there is a need for electroconductive compositions which are highly conductive and which have limited chemical reactivity with the underlying surface layers of the PERC solar cell substrate. In particular, electroconductive compositions for use in forming backside soldering tabs on a PERC solar cell that do not damage the rear passivation or capping layers and which improve electrical performance of the cell are desired.